Really cute and faithful. The entire movie is very well designed, and it's got a good plot to it. I remember when Charlie Brown didn't learn his lesson about the football, but I didn't think it was going to make it to the movie, until I get to the credits and I saw it. :)
Also, I didn't know about The Little Red Haired Girl (and for the rest of the review, I'm just going to call her Heather, because that's her original name). I thought she was new, and unlike in the comics, she looks way more beautiful than she did. I think Blue Sky Studios did a fantastic job on making her.
One thing it lacks, however, is the original cartoony style (not the kind in modern Peanuts TV series, but the kind in movies like A Charlie Brown Christmas and It's Flashbeagle Time, Charlie Brown). I always loved that feature back when those two movies were released. That part gives off that nostalgic feeling to me. I'm sad it never made it to the movie. :'(
Otherwise, really well polished!